1. Field of the Invention
The invention relates to wireless communications, and more particularly, to a system and method for prioritizing signaling messages that are multiplexed into a digital voice or data transmission channel to reduce the impact of the signaling transmission on voice quality.
2. Description of the Related Art
Wireless communication systems generally are comprised of many mobile stations (also referred to as user terminals) and one or more base stations. A base station will provide service to several mobile users via individual communications channels.
A typical terrestrial wireless communications system comprises at least one terrestrial base station and a number of mobile stations (for example, a mobile telephone). In a satellite communications system, links between a mobile station (MS) and a base station (BS) or gateway (GW) are provided by one or more relay satellites. A base station can link a mobile station to other mobile stations or to a terrestrial network, such as a public telephone system.
A variety of multiple access communications systems have been developed for transferring information among a large number of system users. Three known techniques employed by such multiple access communications systems include time division multiple access (TDMA), frequency division multiple access (FDMA), and spread-spectrum modulation techniques, such as code division multiple access (CDMA) spread-spectrum techniques, the basics of which are well known in the art.
The use of CDMA techniques in a multiple access communications system is disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990, titled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters,xe2x80x9d and U.S. Pat. No. 5,691,974 which issued Nov. 11, 1997, titled xe2x80x9cMethod And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy,xe2x80x9d which are both commonly assigned with the present invention, and are incorporated herein by reference.
The above-mentioned patent documents disclose multiple access communications systems in which a large number of generally mobile or remote system users each employ at least one mobile station to communicate with other system users or users of other connected systems, such as a public telephone switching network. The mobile stations communicate through base stations using CDMA spread-spectrum type communications signals.
In a typical spread-spectrum communications system, a set of preselected pseudo random noise (PN) code sequences is used to modulate (i.e., xe2x80x9cspreadxe2x80x9d) information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communications signals. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal. In a xe2x80x9cforwardxe2x80x9d communications link (that is, in a communications link originating at a base station and terminating at a mobile station), PN spreading codes are used to discriminate between signals transmitted by a base station over different beams, and to discriminate between multipath signals. These PN codes are typically shared by all communications signals within a beam.
In a typical CDMA spread-spectrum system, channelizing codes are used to discriminate between signals for particular mobile stations transmitted within a satellite beam on the forward link. That is, a unique orthogonal channel is provided for each mobile station on the forward link by using a unique xe2x80x9cchannelizingxe2x80x9d orthogonal code. Signals intended for conveying information to particular mobile stations are referred to as xe2x80x9ctraffic channels.xe2x80x9d
Typical CDMA spread-spectrum communications systems, such as disclosed in U.S. Pat. No. 4,901,307, contemplate the use of coherent modulation and demodulation for forward link mobile station communications. In communications systems using this approach, a xe2x80x9cpilotxe2x80x9d carrier signal (hereinafter referred to as a xe2x80x9cpilot signalxe2x80x9d) is used as a coherent phase reference for base station-to-subscriber links. That is, a pilot signal, which typically contains no data modulation, is transmitted by a base station throughout a region of coverage. A single pilot signal is typically transmitted by each base station for each frequency used. These pilot signals are shared by all mobile stations receiving signals from the base station.
Pilot signals are used by mobile stations to obtain initial system synchronization and time, frequency, and phase tracking of other signals transmitted by the base station. Phase information obtained from tracking a pilot signal carrier is used as a carrier phase reference for coherent demodulation of signals transmitted on the traffic channels. This technique allows many traffic channels to share a common pilot signal as a phase reference, providing for a less costly and more efficient tracking mechanism. As noted above, a traffic channel is referred to as a channel carrying signals for a particular mobile station.
When a mobile station is not involved in a communications session (that is, the mobile station is not receiving or transmitting user traffic), the base station can convey information to that particular mobile station using a channel known as a paging channel. For example, when a call has been placed to a particular mobile phone, the base station alerts the mobile phone by means of a signal transmitted on the paging channel. Paging channels are also used to distribute system overhead information.
A mobile station can respond to a paging signal by sending a signal over a channel referred to as an xe2x80x9caccess channelxe2x80x9d on the xe2x80x9creversexe2x80x9d communications (that is, the communications link originating at the mobile station and terminating at the base station). The access channel is also used by a mobile station to originate a communication link or call, or send location updating information. A particular access channel is typically associated with a particular paging channel.
Traffic channels include voice data packets (sometimes called voice data xe2x80x9cstreamsxe2x80x9d) and signaling messages carrying control information and short text messages specific to a mobile station. Voice packets and signaling messages share a single traffic channel. The traffic channel comprises frames that are used to transmit either voice packets or signaling messages (signaling data packets).
Conventional digital voice coding techniques (vocoders) cause single words to be transmitted during several separate traffic channel frames. An artifact of this approach is that degradation of voice reception occurs if voice data frames (notably voice frames including accented syllables) are xe2x80x9cblanked,xe2x80x9d or deleted, in order to transmit critical signaling messages (i.e., those required to maintain the link with the mobile station). The random timing and variable length of critical signaling messages results in voice packets being deleted from the traffic channel data stream for an unpredictable number of traffic channel frames.
In a system that uses a variable data rate vocoder, for typical speech patterns approximately half of the transmitted traffic channel frames are sent at a lower data rate. Special advantage can be taken by delaying signaling messages and multiplexing them into the traffic channel data stream during the lower rate voice frames. These lower rate frames do not occupy the full bandwidth of the traffic channel, and the spare bandwidth can be used to carry signaling messages without affecting voice quality.
What is needed is a technique to efficiently transmit voice packets and signaling messages without degrading voice reception by reducing the impact of the above mentioned unpredictability.
One purpose of the invention is to prioritize signaling messages by positioning signaling message frames within a traffic channel to reduce their impact on voice quality. In other words, the invention places signaling message frames within the traffic channel so as to avoid unnecessary deletion of voice information. Because air links are bandwidth-limited by nature, arbitrary voice frame deletions would adversely affect voice signal quality. For example, voice quality will be seriously degraded if arbitrarily placed signaling messages made up more than five percent of the traffic channel frames. According to the present invention, signaling messages using up to fifteen percent of the total traffic channel bandwidth will not be perceivable to the human ear.